Although police feared he was heavily armed, the suspect in the Boston Marathon bombings had no firearms when he came under a barrage of police gunfire that struck the boat where he was hiding, according to multiple federal law enforcement officials.

Authorities said they were desperate to capture Dzhokhar Tsarnaev so he could be questioned. The FBI, however, declined to discuss what triggered the gunfire.

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Other law enforcement officials said the shooting may have been prompted by the chaos of the moment and some action that led the officers present to believe Tsarnaev had fired a weapon or was about to detonate explosives.

These new details emerged as investigators continued their examination of the movements and motives of the 19-year-old and his brother in last week's coordinated bombing, which killed three people and wounded more than 250.

Tamerlan Tsarnaev, 26, was killed in a confrontation with police in the early morning hours Friday, four days after the marathon bombings. A transit police officer was seriously wounded in the exchange, in which more than 200 rounds were fired and the suspects lobbed homemade explosives at police. Dzhokhar Tsarnaev escaped and was the subject of a massive manhunt. He was cornered hiding in a boat in the driveway of a house in Watertown, Mass., on Friday evening.

Law enforcement officials described the 30 minutes before the arrest of Tsarnaev as chaotic. One characterized it as "the fog of war" and said that in a highly charged atmosphere, one accidental shot could have caused what police call "contagious fire."

Officers from several agencies gathered around the Watertown house as darkness fell. The FBI was in charge of the scene, but there also were officers from the Massachusetts State Police, local police and transit police.

"They probably didn't know whether he had a gun," said one law enforcement official. "Hours earlier he and his brother had killed a police officer, shot another officer and thrown explosives out of their cars as the police were chasing them. They couldn't assume that he did not have a gun and more explosives."

The FBI declined to discuss the exact sequence of events that led officers to open fire on Tsarnaev's hiding place and whether the dozens of bullets that struck the boat caused any of his gunshot wounds.

A spokesman for the FBI said law enforcement agents were tracking an extremely dangerous suspect who had used guns and explosives on a public street to avoid arrest.

"Law enforcement was placed in an extraordinarily dangerous situation," said FBI spokesman Paul Bresson. "They were dealing with an individual who is alleged to have been involved in the bombings at the Boston Marathon. As if that's not enough, there were indications of a carjacking, gunfire, an ambushed police officer and bombs thrown earlier. In spite of these extraordinary factors, they were able to capture this individual alive with no further harm to law enforcement. It was a tremendously effective outcome under dire circumstances."

Early Friday in Watertown, the brothers engaged in a firefight with police. Tamerlan Tsarnaev was shot and fell to the ground, according to police and photos, and Dzhokhar Tsarnaev climbed back in a Mercedes SUV carjacked earlier. He drove at police and struck his wounded brother on the street. Tamerlan Tsarnaev, who was dragged a distance by the car, was declared dead on arrival at a Boston hospital.

A criminal complaint filed in federal court in Massachusetts on Monday to support charges against Dzhokhar Tsarnaev said police recovered a single firearm and half a dozen explosives and homemade bombs from the scene of the shootout.

In the ensuing chaos, Tsarnaev accelerated away, abandoned the car and eventually made it on foot just beyond a cordon quickly set up by police. Around 6 p.m. Friday, Tsarnaev was detected hiding beneath a plastic cover on a boat by its owner, who called in police. A thermal imaging unit in a police helicopter confirmed a presence in the boat.

"You can't second-guess what they were doing on that scene," said a second law enforcement official. "Their own lives were in danger."

In the immediate aftermath of Tsarnaev's capture, police officials said he had fired from the boat and he was reported to have been captured with several weapons. There were also reports that the gunshot wound he suffered to the throat might have been an attempt to kill himself as police moved in.

Tsarnaev continues to be treated in a Boston hospital, where his condition has been upgraded from critical to fair. He began communicating in writing and some speech with a special team of FBI interrogators Saturday night and was officially charged Monday.

Thousands of MIT students and police officers from across the United States attended a memorial service on the grounds of the Massachusetts Institute of Technology to remember the 27-year-old police officer.

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